天然气水合物储层测井评价及其影响因素

天然气水合物储层准确识别和精细定量评价是自然界水合物开发利用和环境影响研究的基础,而地球物理测井则是除地震和钻探取心外最有效的原位识别和评价方法。电阻率和声波测井是最早应用于水合物储层识别的井内地球物理方法,由于具有较高的准确性和可靠性,二者仍是目前水合物测井的主要方式,并不断衍生出新的技术,如环电阻率、方向电阻率、多极声波测井等;同时,其他如井内成像、密度、电磁波、核磁共振等测井方法也被综合用于识别水合物储层。经过最近20年的发展,水合物系统测井方法已从单一的测井识别发展到运用各种先进的随钻和电缆测井评价复杂地质条件下水合物储层物性的阶段,初步建立了一套基于常规油气系统的水合物系统测井评价理论体系。该套体系对均质孔隙填充型砂质水合物储层具有较好的适用性,但对于非均质性泥质储层(如裂隙、薄层、互层、富泥质)及胶结或骨架支撑形式的水合物储层应用会产生较大的误差。此外,钻井过程中井眼冲蚀和钻井液侵入等外部因素也会影响水合物测井识别和评价结果。通过了解不同测井方法及其最新的研究进展,认为未来水合物测井技术的发展应侧重提高水合物地层识别和评价的准确性,开发复杂条件下水合物储层岩石物理模型,并综合利用地震、测井、岩心分析等方法修正所建立的岩石物理模型和校正实际的测井结果。

Well logging assessment of natural gas hydrate reservoirs and relevant influential factors

The accurate identification and quantitative assessment of natural gas hydrate reservoirs are thought to be a base for development and utilization of gas hydrates and study of environmental impact. Besides seismic prospecting and core drilling, geophysical well logging is one of the most effective method of identification and evaluation to understand the nature of hydrate reservoirs. Resistivity and acoustic logging as the earliest applied geophysical methods in the identification of hydrate reservoirs are still ways for hydrate well logging due to their relatively good accuracy and reliability and therefore derive some new well logging methods such as ring resistivity, directional resistivity, multiple acoustic logging, etc. Other logging methods including borehole imaging, density, electromagnetic wave, nuclear magnetic resonance, etc. are also used together with resistivity and sonic logging to make an integrated interpretation and evaluation of hydrate reservoirs. With development of the past decades, well logging method of hydrate systems has finished a process from using single logging to adopting wireline and advanced logging-while-drilling tools to evaluate the petrophysical nature of gas hydrate reservoirs under complex geological conditions, and a preliminary theory and technology system based on conventional petrophysical models has been established. This system is generally suitable for homogeneous hydrate reservoirs, but not for heterogeneous argillaceous hydrate reservoirs (e.g. fracture, thin layer, interbed, shale-rich sediment, etc.). In addition, some external factors, such as wellbore washout and invasion of drilling fluid will also affect results of well logging. The present paper reviews the latest research progress in different well logging methods, and suggests that in the near future, new petrophysical models for identification and evaluation of complicated heterogeneous hydrate reservoirs as well as calibration methods for compensating defect caused by wellbore washout and invasion of drilling fluid in well logging will be set up or adopted to enhance accuracy of well logging in identification and evaluation of hydrate reservoirs.